1. Field of the Invention
The present invention is directed to improved localization of targets in the endfire beam generated by a line array of receivers. This is done with artificial Doppler shifts created by electronically moving a single receiver along the aperture.
2. Description of the Prior Art
Direction finding devices are used to determine the direction of propagating signals and hence the location of the source of these signals. These devices are utilized in spectrum monitoring, reconnaissance and surveillance. Direction finders utilize arrangements or arrays of receivers to detect arriving signals and to determine the direction from which these signals originated. For example, appropriate time delays applied to the receivers serves to steer the array and to calculate the direction of arrival of the impinging signals. Therefore, the data gathered from the receivers are used in conjunction with data processing algorithms to interpret the data and determine signal propagation direction relative to the receivers.
Some applications utilize the properties of Doppler effects to analyze the data obtained from the receivers. The Doppler effect or Doppler shift expresses the apparent change in the frequency and wavelength of a wave perceived by an observer that is moving relative to the source of the wave. This relative motion can be caused by the movement of the source, the observer or both the source and the observer. For example, an observer or receiver having velocity vr relative to a source having velocity vs introduces a Doppler shift as follows:
                              f          d                =                  f          ⁢                                                    c                +                                  v                  r                                                            c                -                                  v                  s                                                      .                                              (        1        )            The Doppler shift can be derived for a plane wave from a stationary source having a pressure field of the form p(x,t)=P0ei(ωt−kx). (A stationary receiver at x=0 measures the field p(x,0)=P0eiωt, while a receiver moving according to x=−vt, i.e., opposite to the propagation direction of the plane wave, measures the field p(x,t)=P0ei(ω+ωv/c)t=P0eiωt(1+v/c).
One direction detection device applications employing Doppler effects uses a rotating structure possibly containing multiple circular arrangements of receivers. The Doppler shift is affected by whether or not a given rotating antenna is moving toward or away from the propagation direction of the signal. U.S. Pat. No. 3,490,023 discloses systems where a Doppler shift between the signal and the receiver is induced by physically moving or rotating a detector. However, the amount of movement and hence the sensitivity of the device is limited by the physical limitations of moving the detector. U.S. Pat. No. 3,845,487 discloses a system where linear motion of a single receiver is simulated using a single detector. These systems, however, are limited in the types of signals that can be processed, the amount of Doppler shift that can be induced and the accuracy or resolution with which the direction of the signal can be detected.